More than 1,600 genes encoding transcription factors have been identified in the Arabidopsis genome sequence, but their physiological functions are not yet fully understood. In this study, a small subfamily of single-MYB transcription factor genes, designated RSM1, RSM2, RSM3 and RSM4 (RADIALIS-LIKE SANT/MYB 1-4), was characterized. Here, we mainly examined the RSM1 gene, and found that it appears to play a role in close connection with the HLS1 (HOOKLESS 1) gene during early morphogenesis. Etiolated seedlings overexpressing RSM1 showed several phenotypes similar to those of hls1-1, viz., lack of apical hooks with short hypocotyls, and a defect in gravitropism. Furthermore, both RSM1-ox and hls1-1 seedlings were hypersensitive to red light during early photomorphogenesis, displaying shorter hypocotyls than wild-type seedlings. The histochemical profile of the DR5::GUS auxin-reporter in the RMS1-ox seedlings was considerably different from that in the wild-type seedlings. These results are discussed in the context of the possibility that RSM1 is implicated in HLS1-mediated auxin signaling, which is responsible for regulation of the early photomorphogenesis of Arabidopsis thaliana.